Excellent question. Hard to find a straight answer just by googling.

Question, if you go to:
- Control Panel
- Hardware and Sound
- View Devices And Printers

Are both scanners listed there? Or only the most-recently-installed one?

If they are both listed there, then maybe it's just a question of right-clicking on the desired one and selecting "Set as default" and then using that one.

If they're not both listed there, then you might be right: Maybe you can only have one TWAIN USB scanner at a time.

Seems like something they would have fixed in Windows 10 by now, but maybe not.